Lead frame chip scale package

ABSTRACT

A method for producing chip scale IC packages includes the step of mounting a lead frame panel on a temporary support fixture in order to provide support and protection during the manufacturing process. An embodiment of the temporary support fixture includes a sheet of sticky tape secured to a rigid frame. The rigid frame maintains tension in the sheet of sticky tape to provide a stable surface to which the lead frame panel can be affixed. Installation of IC chips and encapsulation in protective casings is performed as in conventional IC package manufacturing. If encapsulant material is to be dispensed over the IC chips, an encapsulant dam can be formed around the lead frame panel to contain the flow of encapsulant material. The temporary support fixture can be used in any IC package manufacturing process in which lead frames require supplemental support.

BACKGROUND INFORMATION

1. Field of the Invention

The present invention relates to integrated circuit packages, and morespecifically, to the production of a chip scale integrated circuitpackage using a lead frame.

2. Related Art

An integrated circuit (IC) package encapsulates an IC chip, or die, in aprotective casing and also provides power and signal distributionbetween the IC chip and an external printed circuit board (PCB). Acommon IC package uses a metal lead frame to provide the electricalpaths for that distribution. For production purposes, a lead frame panel110 made up of multiple lead frames 120 is etched or stamped from a thinsheet of metal, as shown in FIG. 1a. An IC chip 130 is then mounted andwire bonded to each lead frame 120, as shown in FIG. 1b. Wire bonding istypically performed using fine gold wires 140. Each IC chip 130 is thenencapsulated in a protective casing 160, which can be produced byinstalling a preformed plastic or ceramic housing around each IC chip130, or by dispensing and molding a layer of encapsulated material overall IC chips 130. Next, lead frames 120 are cut apart, or singulated,and multiple electrical interconnections are attached to the lead framein order to produce individual IC packages 190, as shown in FIG. 1c. Theelectrical interconnections provide the electrical interface between ICpackage 190 and the external PCB, and can take a variety of forms. FIG.1c depicts a lead frame ball grid array (BGA) IC package in whichelectrical interconnections are provided by solder balls 150 mounted onthe bottom surface of lead frame 120. Other types of electricalinterconnections can be seen in FIG. 1d, which shows examples of commonIC packages using lead frames, including a small outline package (SOP)191, a pin-through-hole (PTH) package 192, and a plastic leaded chipcarrier (PLCC) 193.

The use of a lead frame provides an inexpensive means for IC packagemanufacturing. Etching or stamping a sheet of thin metal to produce thedesired lead frame patterns is a well-known manufacturing process, andis conducive to high-volume, low-cost production. In addition, the leadframe panel provides a support framework for the IC chips during ICpackage assembly. However, as IC chip device densities increase and ICpackage sizes decrease, the geometries used in the electricalcommunication paths between the IC chip and the PCB decrease. Forexample, a chip scale package requires that the protective casing be nomore than 20% larger than the IC chip. As a result, the area availablefor the electrical paths provided by the lead frame is significantlyreduced, demanding much finer geometry, the lead frame thickness must bereduced to a point where the lead frame panel rigidity would no longerbe sufficient to provide the necessary support during the IC packageassembly process. Also, the fragile lead frame patterns would be moresusceptible to damage during the manufacturing process. As a result,chip scale IC packages must use more costly techniques such as tapeautomated bonding (TAB) or printed substrate backing.

FIGS. 1d-1 through 1d-3 show examples of common IC packages using leadframes. FIG. 1d-1 shows a small outline package (SOP) 191. FIG. 1d-2shows a pin-through hole (PTH) package 192. FIG. 1d-3 shows a plasticleaded chip carrier (PLCC) 193.

According, it is desirable to provide an IC packaging method that allowsthe use of a lead frame in a chip scale package.

SUMMARY OF THE INVENTION

The present invention provides a method for producing chip scale ICpackages using lead frames. A temporary support fixture provides supportand stability to a thin lead frame panel having the fine geometriesrequired for higher-density IC chip interfaces. An embodiment of thesupport fixture includes an adhesive pad made of one-sided sticky tapemounted to a rigid frame made of stainless steel, the rigid framemaintaining the adhesive pad in a fixed configuration providing a stableflat surface for support of the lead frame panel. Alternatively, therigid frame and adhesive pad can be made of any materials compatiblewith the IC package manufacturing process and capable of supporting thelead frame panel through the manufacturing process. The adhesive pad canalso be patterned to case the manufacturing process. The rigid frame caninclude positioning features to assist in the alignment of the leadframe and adhesive pad. If encapsulant material is to be dispensed overthe lead frame panel, a containment dam can be formed around the leadframe after it is installed on the adhesive pad, to provide a boundaryfor encapsulant material flow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a representation of a typical lead frame panel inaccordance with one embodiment of the present invention;

FIG. 1b shows athe lead frame panel of FIG. 1apopulated with IC chips;

FIGS. 1c-1 and 1c-2 shows bottom and cross sectional views of a signalleadframe BGA IC package;

FIGS. 1d-1 through 1d-3 shows examples of common IC packages;

FIG. 2a shows an embodiment of a rigid support fixture;

FIG. 2b shows a lead frame panel mounted on an embodiment of a supportfixture;

FIGS. 3a and 3b show a flow diagram of a manufacturing process using atemporary support fixture

Use of the same reference number in different figures indicates similaror like elements.

DETAILED DESCRIPTION

Generally, an integrated circuit (IC) package encapsulates an IC chip,or die, in a protective casing and also provides power and signaldistribution between the IC chip and an external printed circuit board(PCB). A metal lead frame can be used to provide the electrical pathsfor that distribution. A lead frame panel suitable for use in accordancewith the present invention is illustrated in FIG. 1a. For productionpurposes, a lead frame panel 110 made up of multiple lead frames 120 isetched or stamped from a thin sheet of metal, as shown in FIG. 1a. An ICchip 130 is then mounted and wire bonded to each lead frame 120, asshown in FIG. 1b. Wire bonding is typically performed using fine goldwires 140. Each IC chip 130 is then encapsulated in a protective casing160 which may be formed by dispensing and molding a layer of encapsulantmaterial over all IC chips 130. Next, lead frames 120 are cut apart, orsingulated to produce individual IC packages 190.

As seen in FIG. 1a, the panel 110 includes a two dimensional array ofdevice areas. Each device area has a plurality of contacts 112 and a dieattach pad 114. The panel has a grid of tie bars 115 that extend inperpendicular rows and columns to define the device areas. The tie bars115 carry the contacts 112 and die attach pads 114.

The present invention embodiment shown in FIG. 2a employs a rigidsupport fixture during the manufacturing process to enable the use oflead frames in chip scale IC packages. An embodiment of a supportfixture 200 includes a rigid frame 210 and an adhesive pad 220, as shownin the exploded isometric diagram of FIG. 2a. Because pad 220 is affixedalong its border to frame 210, it maintains sufficient tension toprovide a stable supporting surface for a lead frame panel 110. Bymaking pad 220 out of a thin, flexible, and electrically non-conductivematerial, it provides a support structure that will not interfere withthe conventional manufacturing processes used in IC package assembly.The size of the interior opening of frame 210 is large enough to allowlead frame panel 110 to be fully supported by pad 220. Multiple IC chips130 are then installed and wire bonded on lead frame panel 110, as shownin FIG. 2b. Subsequent encapsulation of IC chips 130 in protectivecasings proceeds as in conventional lead frame processing. If a moldedprotective casing is to be applied, an encapsulant dam 240 can beconstructed around the perimeter of lead frame panel 110. Dam 240 can bemade of any substantially rigid substances, including premolded plastic,epoxy, or tape, and serves to prevent flow of encapsulant materialbeyond the boundaries of lead frame panel 110. Alternatively, containingmeasures for encapsulant material could be incorporated into thedispensing mechanism. Once encapsulation is complete, support fixture200 can be removed, either before or after singulation.

The embodiment of the present invention shown in FIGS. 2a and 2b can beconstructed from common and readily available materials. Pad 220 can bemade from a 3M or Nitto-brand sticky tape used in conventional wafer sawoperations. Likewise, a stainless steel ring of the type used inconventional wafer saw operations can be employed for frame 210.However, both pad 220 and frame 210 can be implemented in many differentways as well. For example, frame 210 can be constructed from any rigidmaterial compatible with the IC package assembly process, such ascopper, aluminum, or even non-metals such as ceramic or plastic. Also,while depicted as a thin circular element, frame 210 can also take avariety of configurations depending on handling, interface, and userrequirements. For instance, frame 210 can include positioning featuresto ensure consistent alignment for lead frame panel 110 and adhesive pad220. A circular outline for frame 210 provides compatibility withconventional handling requirements for IC production, but is notrequired aspect of the present invention.

Similarly, numerous implementations of adhesive pad 220 are possible.Any material compatible with the IC package assembly process and capableof providing the necessary support to the lead frame panel and IC chipscan be used. The sticky tape mentioned previously is a convenient choicedue to widespread current usage and availability. The use of one-sidedsticky tape enables pad 220 to be applied to the bottom surface of frame210 and provide an adhesive surface for mounting of lead frame panel110, without requiring additional attachment materials or components.Pad 220 can also be patterned by removing selected portions in order tofacilitate subsequent assembly operations such as electricalinterconnection formation. Removal of pad 220 once packaging is completecan be performed in various ways, depending on the nature of theadhesive material used. A light adhesive material may allow pad 220 tosimply be peeled away from frame 110. An alternative bonding agentrequires exposure to UV light before removal of pad 220 can take place.

FIGS. 3a and 3b show a graphical flow chart illustrating a method formanufacturing a lead frame BGA package using an embodiment of thepresent invention. The manufacturing process is described in conjunctionwith the elements described in FIGS. 2a-2c. In a step 310 in FIG. 3a,adhesive pad 220 is applied to rigid frame 210 to create support fixture200. Lead frame panel 110 is then mounted on pad 220 in a step 320. Anoptical step 330 allows encapsulant dam 240 to be applied around theborder of lead frame panel 110 if subsequent encapsulant materialdispensing is to be performed. Next, an IC chip 130 is mounted and wirebonded onto each of the lead frames 120 of lead frame panel 110.Continuing the process in FIG. 3b, a step 350 involves dispensing aportion of encapsulant material 170 into the area defined by dam 240 tocover IC chips 130, and then curing material 170 to a desired hardness.In a step 360, pad 220 is removed from lead frame panel 110. Next, in astep 370, a wafer saw operation is performed to singulate lead framepanel 110 into individual IC packages. The singulation process convertsthe layer of hardened encapsulant material 170 into individualprotective casings 160. Finally, in a step 380, solder balls 150 areapplied to desired electrical interconnection locations to complete leadframe BGA IC package 190.

In this manner a lead frame BGA IC package can be produced using atemporary support structure. This enables the production of IC packagesusing lead frames that would otherwise be too fragile to withstandconventional manufacturing processes. It should be noted that whileparticular embodiments of the present invention have been shown anddescribed, it will be apparent to those skilled in the art that manymodifications and variations thereto are possible, all of which fallwithin the true spirit and scope of the invention. For example, thewafer saw operation of step 370 can be performed prior to removal ofsupport fixture 200 from lead frame panel 110. Also, solder balls 150could be applied to lead frames 120 in step 370 prior to singulation.Alternatively, appropriately located openings in adhesive pad 220 wouldallow solder balls 150 to be applied without removing pad 220. Certainlead frame designs may even allow patterning of pad 220 such thatremoval is unnecessary. Finally, while the present invention has beendescribed with reference to chip scale IC package manufacturing, it canbe applied to any IC package manufacturing process involving leadframes, including non-chip scale and non-BGA IC packages such as SOP's,PLCC's, and PTH packages.

1. A fixture for providing temporary support to a lead frame during anIC package manufacturing process comprising an adhesive pad attached toa rigid frame, said rigid frame having an opening larger than said leadframe so as to accommodate said lead frame entirely within an outerperiphery of said rigid frame, and such that said adhesive pad is heldin tension by said rigid frame to provide a stable support surface forsaid lead frame.
 2. The fixture of claim 1 wherein said adhesive pad isconstructed of single-sided sticky tape.
 3. A fixture for providingtemporary support to a lead frame during an IC package manufacturingprocess comprising an adhesive pad attached to a rigid frame such thatsaid adhesive pad is held in tension by said rigid frame to provide astable support surface for said lead frame, wherein said rigid framecomprises a stainless steel ring having an opening of a diameter largerthan said lead frame.
 4. The fixture of claim 1 wherein said rigid frameincludes a set of positioning features for positioning and aligning saidlead frame and said adhesive pad with respect to said rigid frame. 5.The fixture of claim 1 wherein said adhesive pad is patterned tofacilitate a subsequent manufacturing step.
 6. A fixture as in claim 1,wherein said lead frame includes multiple die attach pads foraccommodating multiple integrated circuit dies.
 7. A fixture as in claim1, further comprises an encapsulation dam having an outer wall adaptedfor enclosing said lead frame.
 8. A fixture as in claim 1, wherein eachdie attach pad is provided around its periphery multiple leads forwire-bonding.
 9. A fixture as recited in claim 1, wherein the rigidframe is formed from stainless steel.